This invention relates to medical diagnostic equipment and methods and is particularly concerned with hollow viscus tonometry.
Stress ulceration and intestinal ischemia are two serious problem that plague physicians involved in the management of patients in intensive care units. Intestinal ischemia, in particular, has an insidious onset and may not be detected until days after the intestine has become gangrenous. A delay in the diagnosis of intestinal ischemia may have devastating consequences for a patient. The availability of means for early diagnosis and management of patients with these problems would have immediate applicability in all intensive care units, especially where the procedure can be conveniently conducted with reasonable safety and reliability.
It has been established that a fall in the intramuscosal pH may precede the development of stress ulceration. One aspect of the invention involves the discovery, in the laboratory, that a fall in intrasmucosal pH also occurs within minutes of inducing intestinal ischemia in dogs. The fall in pH in intestinal mucosa, and hence the liklihood of stress ulceration or ischemia, can be reliably calculated from a pCO.sub.2 (partial pressure of CO.sub.2) in luminal fluid and the bicarbonate concentration in arterial blood. The method of calculating the pH in intestinal mucosal tissue, pursuant to principles of the invention, has been validated by direct measurements under a variety of conditions simulating clinical problems. A correlation coefficient in the order of 0.92 to 0.95 has been obtained in each of sixteen dogs. It will be readily recognized that the validity of the procedure is inherently extensible to humans.
To measure the pCO.sub.2 in the lumen of the gut it is necessary to obtain a sample of fluid that has been in contact with the wall of the gut for a certain time period, usually at least half an hour. It is difficult to aspirate fluid from the lumen of the gut with any consistency, for any fluid instilled into the lumen passes into distal and proximal regions. It is much easier to obtain samples from the stomach, but samples obtained from the stomach frequently contain foreign material that can damage a gas analyzer.
A particular aspect of the invention involves the creation of a new and unique catheter via which the desired sample or samples can be obtained without the complications of prior techniques. One embodiment of the new and unique catheter comprises a catheter tube having a walled sampling chamber on the tube with the sampling chamber being in communication with the hollow interior of the tube. The wall of the sampling chamber comprises a material which is substantially impermeable to liquid yet is highly permeable to gas. One suitable material is polydimethylsiloxane elastomer.
In use the catheter is introduced into a patient to place the sampling chamber at a desired site within the organ of interest. An aspirating liquid fills the interior of the sampling chamber. The sampling chamber is left in place at the desired sampling site long enough to allow the gases present to diffuse through the wall of the sampling chamber into the aspirating liquid. The time should be long enough for the gases to equilibrate. The liquid impermeable nature of the sampling chamber wall material prevents both the aspirating liquid from leaking out of the chamber and also the intrusion of any liquids into the aspirating liquid. After the appropriate amount of placement time has elapsed the aspirating liquid is aspirated along with the gases which have diffused into it. The sample thus obtained is analyzed for gas content, in particular for pCO.sub.2. In this way the pCO.sub.2 within the lumen of the gut can be reliably measured with the fluid being free from lumenal debris.
In carrying out the diagnostic method of the invention the pCO.sub.2 measurement is utilized in conjunction with a measurement of the bicarbonate concentration in an arterial blood sample of the patient for determining the pH of the tract wall.
Depending upon the particular condition of a given patient, the catheter may be left in place and samples may be taken at periodic intervals so that pH values may be periodically calculated. The procedure has a high reliability in accurately diagnosing intestinal ischemia in its incipient stages and such detection can be useful in treating the patient so that the potentially devestating consequences resulting from less timely detection may often be avoided.
The invention has applicability to many hollow internal organs although in the techniques described in detail herein the invention involves diagnosis within the gastrointestinal tract system. Depending upon the particular site or sites of interest within a patient, different types of catheters embodying principles of the invention may be appropriately used. One embodiment involves a catheter as described above. In that embodiment the catheter has a single sampling chamber and a single walled tube. Another embodiment contemplates the use of multiple individual single sampling chamber catheters of varying lengths bundled together to form a multiple sampling site catheter. Still another embodiment involves the use of a sump-type nasogastric tube. Yet another embodiment comprises a pliable catheter with a mercury bag at its end which may be used for certain procedures.
In use of an embodiment that employs multiple sampling chambers, the pH in intestinal mucosal tissue at one site may be calculated and compared with the calculated pH values at other sites. This analysis can be a useful diagnostic aid to the attending physician. In the case of an abdominal aortic resection a multiple sampling chamber type catheter may be placed intralumenally in series in the colon at the time of the resection, and it may be used to aid in the early detection of colonic ischemia that occurs insidiously in approximately five percent of the patients subjected to this major operation. A multiple sampling chamber embodiment may also be introduced into the small intestine to monitor the pH and hence perfusion of the gut in patients with low flow states. In critically ill patients who require a nasogastric tube, a single sampling chamber embodiment may be incorporated into a conventional nasogastric tube and placed in the patient's stomach.
It is further contemplated that the invention may be practiced in connection with diagnosis of the billary tract, urinary tract and pancreas for monitoring pH and hence perfusion of the associated organs.
In connection with this invention, a preliminary novelty search developed the following U.S. Pat. Nos. 2,470,665; 3,227,154; 3,548,805; 3,572,315; 3,952,730; and 4,168,703, none of which are deemed pertinent to the claims of the present invention.
The foregoing features and benefits of the invention in its several aspects, along with additional features and benefits, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose presently preferred embodiments of catheters which embody principles of the invention and are used in the diagnostic aspects of the invention.